1. Field of the Invention
The present invention relates in general to a numerical control system for a machine tool. More particularly, it relates to a numerical controller for controlling the operation of, preferably, a cylindrical grinding machine in accordance with numerical control information.
2. Description of the Prior Art
Generally, the grinding of a cylindrical workpiece surface is performed in one of various grinding modes such as a size control plunge grinding mode, a dead-stop traverse grinding mode and the like which is chosen taking into consideration the length of the ground surface, the required accuracy of finish, the profiles of portions adjoining opposite ends of the ground surface, and other various conditions. In the case where such grinding operation is controlled by means of a conventional numerical controller, there must be prepared a numerical control program which effects the relative movement between a wheel head and a traverse table of a grinding machine in accordance with a sequence cycle appropriate to a desired grinding mode. The preparation of such a control program is laborious. Particularly, in the case where a workpiece has a number of surfaces to be ground in respectively different grinding modes, the numerical control program prepared therein must be so programmed as to grind the surfaces in the respectively appropriate grinding modes, and this causes the operator to spend a great deal of effort and much time.
As noted above, various grinding modes are used in the conventional grinding of cylindrical workpieces which can be assorted through an analysis of the relative movement between the wheel head and the traverse table draws. For example, the grinding modes which can be chosen for external grinding of cylindrical workpieces or workpiece surfaces are assorted into eight kinds as shown in the following TABLE 1.
TABLE 1 ______________________________________ Symbol Kind of Mode ______________________________________ 1 Size control plunge grinding 2 Dead-stop plunge grinding 3 Opposite-end infeed size control traverse grinding 4 Opposite-end infeed dead-stop traverse grinding 5 Left-end infeed size control traverse grinding 6 Left-end infeed dead-stop traverse grinding 7 Right-end infeed size control traverse grinding 8 Right-end infeed dead-stop traverse grinding ______________________________________
The grinding modes prescribe respective fixed sequence cycles in accordance with which the wheel head and the traverse table are to be moved. Accordingly, under the same technical concept as those known numerical controllers which have been developped for control of drilling, boring and turning operations, an attempt may be made to provide the numerical controller that stores the above-noted fixed sequence cycles and that is capable of controlling the grinding operation in accordance with any designated one of the fixed sequence cycles. In the numerical controller so provided, the preparation of the numerical control program used for grinding a workpiece can be made easily because the numerical control program is completed only by designating one of the fixed sequence cycles and inputting to the numerical controller various control variables or parameters such as feed rate, feed amount, spark-out period of time, the number of traverse times and the like which are necessary for the execution of the designated sequence cycle. However, the control parameters that the operator manually inputs to the numerical controller are not only large in number, but also have to be changed in correspondence to the kind of the designated grinding mode. Therefore, it is also required for the operator to choose the control parameters which are necessary for the execution of the designated sequence cycle while he makes reference to a grinding mode-necessary parameter listing. For this reason, the parameter inputting to the numerical controller is time consuming, and error may be involved in choosing the control parameters, thus resulting in the wrong operation of the grinding machine.
Further, in conventional grinding, it is often the case that the change in workpieces or workpiece surfaces to be ground does not necessarily require changing respective feed rates in rough, fine and finish grinding steps, the number of spark-out times and the like. Particularly, it is a practice that a number of surfaces of a cylindrical workpiece are machined at a preliminary turning process to thereby have almost the same grinding allowances as one another. For the purpose of making the preparation of numerical control programs easy, it is therefore desired to use a common or standard set of control parameters as the data necessary for the numerical control grinding of a number of workpieces or workpiece surfaces.
Additionally, in the case of a workpiece having a number of axially spaced cylindrical surfaces to be ground under numerical control, reference index positions of a traverse table are programmed in relation respectively to the workpiece surfaces, and the traverse table movement during the grinding of each of the workpiece surfaces is controlled to be within an appropriate distance starting from one of the reference index positions which is programmed in relation to the workpiece surface being ground. However, if there exists a machining error in the depth of the workpiece center hole into which a work head center is inserted, the respective positionings of the traverse table to the reference index positions under numerical control do not cause the workpiece surfaces to be presented respectively to those positions each of which has a suitable positional relation to a grinding wheel, and therefore, precise grinding of the workpiece surfaces cannot be accomplished. Particularly, when one or both ends of a workpiece surface adjoin shoulder portions of adjacent workpiece surfaces, there may be effected an interference between the grinding wheel and the shoulder portions, which results in the breakage of the grinding wheel as well as in the damage of the workpiece. This problem can be solved by compensating the reference index positions for the machining error in the depth of the workpiece center hole. However, since for the advantage of using any present table index position as a command table index position, the reference index positions are practically programmed in the form of respective absolute values, such compensation must be carried out with respect to all of the programmed reference index positions, and a great deal of time is spent for such compensation. Accordingly, the necessity of making it possible to carry out such compensation within a short period of time has now been encountered.